Within the field of electrochemistry, there is a well-known type of an electrolytic cell known as a chlor-alkali cell. Basically this is a cell wherein chlorine gas and caustic soda, viz., sodium hydroxide, are produced by passing an electric current through a concentrated salt (brine) solution containing sodium chloride and water. A large portion of the chlorine and caustic soda for the chemical and plastics industries is produced in chlor-alkali cells. The cathodes employed in such chlor-alkali cells are subjected to the corrosive environment of the caustic soda.
Such cells are divided by a separator into anode and cathode compartments. The separator characteristically can be a substantially hydraulically impermeable membrane, e.g., a hydraulically impermeable cation exchange membrane, such as the commercially available NAFION manufactured by the E. I. du Pont de Nemours & Company. Alternatively, the separator can be a porous diaphragm, e.g., asbestos, which can be in the form of vacuum deposited fibers or asbestos paper sheet as are well known in the art. The anode can be a valve metal, e.g., titanium, provided with a noble metal coating to yield what is known in the art as a dimensionally stable anode. One of the unwanted by-products present in a chlor-alkali cell is hydrogen which forms at the cell cathode. This hydrogen increases the power requirement for the overall electrochemical process, and eliminating its formation is one of the desired results in chlor-alkali cell operation.
Fairly recently, attention has been directed in chlor-alkali cell technology to various forms of oxygen (air) cathodes. Such cathodes can result in significant savings in the cost of electrical energy employed to operate chlor-alkali cells. Estimates indicate that there is a theoretical saving of about 25 percent of the total electrical energy required to operate chlor-alkali cells provided that the formation of hydrogen at the cathode can be prevented. In other words, about 25 percent of the electrical energy employed in a chlor-alkali cell is used to form hydrogen at the cathode. Hence, the prevention of hydrogen formation by the reduction of oxygen at the cathode results in significant savings in the cost of electrical power. This is the major benefit of and purpose for oxygen (air) cathodes. Such cathodes, being in contact with the electrolyte caustic soda, are subjected to the corrosive action thereof. Additionally, there are internal stresses and forces produced by the very reactions occurring at the cathode which tend to cause deterioration, break up of the active layer and bleed through of liquid in the wetproofing (backing) layer of such electrodes.
One known form of oxygen (air) cathode involves use of an active cathode layer containing porous active carbon particles whose activity in promoting the formation of hydroxide may or may not be catalyzed (enhanced) using precious metal catalyst materials, such as silver platinum, etc. Unfortunately, however, the pores of such active carbon particles may become flooded by the caustic soda thereby significantly reducing their ability to eliminate the formation of hydrogen at the cathode and resulting in decreased operating efficiency. Various attempts have been made to solve this wettability problem, e.g., by providing a backing layer which is hydrophobic to reduce the likelihood of wetting or flooding of the carbon particles in the active layer by the catholyte liquor. Various forms of polytetrafluoroethylene (PTFE) have been utilized for this purpose. With the use of PTFE, however, comes the problem of reduced electrical conductivity in the cathode active layer in as much as PTFE, per se, is nonconductive. Some oxygen (air) cathodes contain PTFE in both the active layer and in a backing sheet laminated thereto. Such PTFE has been employed in particulate or fibrillated (greatly attenuated and elongated) form to impart hydrophobicity to the desired layer. Thus it can be seen that the development of corrosion-resistant oxygen (air) cathodes of improved durability for use in conjunction with chlor-alkali cells is an overall objective in the field of electrochemistry.
U.S. Pat. No. 4,058,482, Baris et al, discloses an active layer sheet material principally comprised of a polymer such as PTFE and a pore-forming material wherein the sheet is formed of coagglomerates of the polymer and the pore former. This patent teaches mixing polymer particles with positively charged particles of a pore former, e.g., zinc oxide, to form coagglomerates thereof followed by mixing same with a catalyst suspension so as to form coagglomerates of catalyst and polymer-pore-former agglomerates followed by pressing, drying and sintering these coagglomerates. Subsequent to this sintering, the pore former can be leached out of the electrodes.
U.S. Pat. No. 4,150,076 (a division of U.S. Pat. No. 4,058,482) is directed to the process for forming the sheet of U.S. Pat. No. 4,058,482, said process involving formation of polymer-pore-former coagglomerates, distributing same as a layer on a suitable electrode support plate, for example, a carbon paper, to form a fuel cell electrode by a process which includes pressing, drying, sintering and leaching.
U.S. Pat. No. 4,170,540, Lazarz et al, discloses microporous membrane material suitable for electrolytic cell utilization and formed by blending particulate polytetrafluoroethylene, a dry pore-forming particulate material and an organic lubricant. These three materials are milled and formed into a sheet which is rolled to the desired thickness, sintered and subjected to leaching of the pore-forming material.
U.S. Pat. No. 4,177,159 to R. M. Singer discloses formation of a gas electrode by directly filtering a cosuspension of catalyzed or uncatalyzed carbon particles and PTFE on a carbon paper substrate.
U.S. Pat. No. 4,179,350 to G. A. Deborski is directed to porous oxygen electrodes containing a metal phthalocyanine, particulate finely divided metal powder and a pore former, all fabricated together in an integral porous body form of structure. The electrode body can be made in plaque form.
British Pat. No. 1,284,054, Boden et al, is directed to forming an air-breathing electrode containing an electrolyte within an air-depolarized cell. This air-breathing electrode is made by hot pressing a fluoropolymer sheet containing a pore-forming agent onto a catalyst composition (containing silver) and a metallic grid member. According to page 3 of said British patent, the PTFE-pore-forming agent-paraffin wax containing sheet is subjected to a solvent wash to remove the paraffin wax (lubricant and binder) and then sintered in a sintering furnace at the appropriate temperatures for sintering the fluorocarbon polymer. After the PTFE-containing sheet is sintered and while it still contains the pore-forming particles, it is then ready for application to the catalyst composition of the air electrode for the hot pressing operation. Hot pressing involves the use of pressures ranging from about 5,000 to about 30,000 psi in conjunction with temperatures ranging from about 200.degree. F. to 400.degree. F.
U.S. Pat. No. 3,385,780 to I-Ming Feng discloses a thin, porous electrode consisting of a thin layer of a polytetrafluoroethylene pressed against a thin layer of polytetrafluoroethylene containing finely divided platinized carbon, the platinum being present in amounts of 1.2 to 0.1 mg/cm.sup.2 in the electrically conductive face of the thin electrode, viz., the side containing the platinized carbon, i.e., the active layer. A thermally decomposable filler material can be used, or the filler can be a material capable of being leached out by either a strong base or an acid. U.S. Pat. No. 3,385,780 also mentions a single unit electrode involving finely divided carbon in mixture with PTFE.
U.S. Pat. No. 4,135,995 to Cletus N. Welch is directed to a cathode having a hydrophilic portion formed of a solid intercalation compound of fluorine and carbon of the emperical formula CF.sub.x, where x ranges from about 0.25 to 1 and preferably ranges from about 0.25 to 0.7. The intercalation compounds of carbon and fluorine are referred to as hydrophilic, fluorinated graphites and graphite fluorides characterized by an infrared spectrum showing an absorption band at 1220 cm.sup.-1. A layer of hydrophobic material such as polyperfluoroethylene (polytetrafluoroethylene) can be utilized in a hydrophobic portion of the same layer or it can be utilized in the form of a different layer which can be associated with a current carrier layer. The Welch cathode may be utilized as an air (oxygen) cathode.
U.S. Pat. No. 3,838,064 to John W. Vogt et al is directed to a process for dust control involving mixing a finely divided fibrillatable polytetrafluoroethylene with a material which characteristically forms a dust to form a dry mixture followed by sufficient working to essentially avoid dusting. Very small concentrations of PTFE, e.g., from about 0.02 to about 3 percent by weight are employed to achieve the dust control. Corresponding U.S. Pat. No. 3,838,092 also to Vogt et al is directed to dustless compositions containing fibrous polytetrafluoroethylene in concentrations of about 0.02 percent to less than 1 percent, e.g., about 0.75 percent by weight of PTFE based on total solids.
An article entitled "On the Effect of Various Active Carbon Catalysts on the Behavior of Carbon Gas-Diffusion Air Electrodes: 1. Alkaline Solutions" by I. Iliev et al appearing in the Journal of Power Sources, 1 (1976/1977) 35, 46, Elseview Sequoia S. A., Lausanne-printed in the Netherlands, at pages 35 to 46 of said Journal describes double-layer, fixed-zone Teflon-bonded carbon electrodes having a gas supplying layer of carbon black "XC" wetproofed with 35 percent Teflon and an active layer consisting of a 30 mg/cm.sup.2 mixture of the same wetproof material "XC-35" and active carbon "weight ratio of 1:2.5." These electrodes were sintered at 350.degree. C. under a pressure of 200 kg/cm.sup.2 and employed as oxygen (air) cathodes in alkaline test environments.
The publication "Advances in Chemistry Series," copyright 1969, Robert F. Gould, (Editor), American Chemical Society Publications, contains at pages 13 to 23 an article entitled "A Novel Air Electrode" by H. P. Landi et al. The electrode described contains 2 to 8 percent PTFE, is produced without sintering and is composed of graphtic carbon (ACCO Graphite) or metallized graphitic carbon particles blended with a PTFE latex and a thermoplastic molding compound to form an interconnected network which enmeshes the filter particles. This blend is molded into a flat sheet, and the thermoplastic is then extracted.
British Pat. No. 1,222,172 discloses use of an embedded conductive metal mesh or screen (35) within a formed electrode (30) containing a particulate matrix (34) of polytetrafluoroethylene polymer particles (21) in which there are located dispersed electrically conductive catalyst particles (24) which can be silver-coated nickel and silver-coated carbon particles, viz., two different types of silver-coated particles in the PTFE particulate matrix in an attempt to overcome an increase in resistance in the gas diffusion fuel cells to which said British patent is directed.